The present invention relates in general to relative humidity sensors and, more particularly, the invention relates to sensors made from cobalt oxide as the active element. Humidity sensors of this type are also referred to as hygrometers or humistors, as it is their electrical resistivity which changes with humidity.
Procedures for manufacturing cobalt oxide humidity sensors or hygrometers are well known. The starting material is a cobalt oxide powder. As pure CoO powder is very expensive, the starting material is generally a mixture of CoO with some Co.sub.3 O.sub.4, but the latter compound dissociates at about 900.degree.C. so the completed sensor will be essentially CoO, the cobaltous oxide. This compound is stable up to its melting point, which is above 1800.degree.C.
The finely divided powder, preferably minus 325 mesh, is mixed with an inert liquid vehicle and viscosifiers to form a screen-printable paste. A thin layer of the paste is then screen-printed in a desired pattern onto a dielectric, high-temperature resistant substrate, typically a high-alumina ceramic. The screened pattern is then dried and fired in air at a temperature in the range of 1350.degree.C. to 1550.degree.C. Electrodes can be preformed on the substrate, co-fired with the paste, or applied in a subsequent operation. The latter is the more common approach as it is generally desired to have the electrodes in a rather elaborate, interdigitated pattern on the top surface. Conductive inks or pastes (platinum-gold, palladium-gold, etc.) are used in the conventional manner.
Before such a sensor can be put to use, it must be accurately calibrated to determine the change in electrical resistance with relative humidity.
The very high specific resistivity of cobalt oxide, which is 10.sup.6 ohms per square or higher, requires that humidity sensors made therefrom be relatively large in order to produce a suitable output. Typically, prior art humidity sensors of this type are manufactured on one inch square substrates. This is of course much too large to be included in integrated circuit packages.
The desirability of having a humidity sensor within a hermetically sealed semiconductor package is manifest. Essentially all semiconductive devices are humidity-sensitive to a greater or lesser degree. As a result, specifications on hermeticity for packages are most stringent. But, while technology for producing hermetic packages is well developed and tests therefor standarized, the fact that a package is hermetic initially says nothing about whether it will remain so after months or years of service, often under severe conditions of shock and vibration. Further, the failure of a seal in service can now be detected only by a malfunction or failure of the circuit.